"He's an amazing man. After everything he's done, he's still modest, quite self effacing actually. He even likes people to think he's not as smart as he really is. Bottom line: He's an incredibly strong leader who's given more of himself for this program than anyone has given for... well... anything I can imagine."

Despite that and several references to his seemingly less than stellar intellect he received an award for Academic Excellence upon his graduation from the Air Force Academy as a Second Lieutenant. (SG1: "Zero Hour")

His personal history is troubled. At some point, he married Sara O'Neill and together they had a son and named him Charlie O'Neill. Shortly before traveling through the Stargate for the first time, Charlie was killed while playing with his pistol and, in the aftermath of this tragedy, he separated from his wife. (Stargate, SG1: "Cold Lazarus")

O'Neill at the mission briefing. From the Season 1 premiere episode, "Children of the Gods".

As such, O'Neill was recalled to active duty and resumed serving with the United States Air Force, regaining his original rank of Colonel. Hammond later authorized O'Neill, Kawalsky and Ferretti to return to Abydos, along with the scientist CaptainSamantha Carter. On Abydos, O'Neill was able to reunite with Daniel Jackson, however, another attack by Apophis injured Ferretti and saw Skaara and Sha're taken. The team returned to Earth with the gate address of Apophis' homeworld. Under the authorization of Hammond, O'Neill was made the leader of the team SG-1 with Carter and Jackson. They were sent off to Chulak, the homeworld of Apophis, along with SG-2 under the command of Kawalsky. SG-1 was captured by Apophis and nearly executed until O'Neill convinced Apophis' First Prime, Teal'c, to join him. O'Neill was able to acquire a staff weapon and blast a hole in the cell, and his team, along with the prisoners, were able to escape back to Earth. (SG1: "Children of the Gods")

While exploring P3R-272, the Ancient's Repository of knowledge was downloaded into his brain. However, Human physiology was not evolved to the point where his brain could handle that information, causing him to begin speaking Ancient and begin to die. However, his subconscious mind was able to write new addresses into the Stargate map, as well as build a generator that allowed him to gate to Othala, the Asgard homeworld. There, the Asgard erased the knowledge from his memory, told him of the Alliance of Four Great Races, and beginning the Asgard's long interest in O'Neill. (SG1: "The Fifth Race")

While on Edora, the Stargate was buried in a meteor shower, and O'Neill was trapped on the planet for over three months. There, O'Neill became close to a local woman, Laira, but after Stargate Command was able to make contact with Edora, he returned to Earth, after MajorSamantha Carter spent three months building a Particle accelerator in a plan to bring him back to the SGC. He promises Laira that he will return someday; however, this never occurs. Laira also holds her belly as he leaves, implying that she is pregnant; however, this is thought to be unlikely, considering the issue is never revisited. It is also thought that Carter comes to terms with her feelings for him during this time, given her reactions to the situation. (SG1: "A Hundred Days")

Shortly afterwards, the Asgard and the Tollans approached Stargate Command with evidence that Earth had been stealing their technology, as well as the technology of the Nox and the Tok'ra. O'Neill and Major GeneralGeorge S. Hammond convinced them that the SGC wasn't responsible, but the Asgard insisted that O'Neill discover who was responsible. O'Neill "stole" a Tollan weapons scanner, and "resigned" from the military. He was approached by ColonelHarold Maybourne, who offered him a chance to acquire alien technology illegally. O'Neill, however, sent word of the Rogue N.I.D. Base to the Asgard, who then retook the technology. Rather than risk being taken by the Asgard, the rogue team follows O'Neill back through the Stargate to the SGC, where they are arrested. Hammond and O'Neill then reveal the plan to the rest of SG-1, who are somewhat offended that they weren't in on it. (SG1: "Shades of Grey")

When the Replicators took control of the Biliskner, Thor was forced to beam O'Neill to the vessel, to give him warning of the threat and help him destroy the ship. O'Neill requested a large amount of ordinance and weapons, and MajorSamantha Carter and Teal'c beamed aboard as well. When Thor revealed that there was no way to destroy the ship easily, Carter came up with the idea to destroy it with an uncontrolled reentry. SG-1 beamed the Stargate to the ship, and was able to gate out before impact. (SG1: "Nemesis", "Small Victories")

At the insistence of the Tok'ra Anise, O'Neill, MajorSamantha Carter, and Dr.Daniel Jackson used the Atanikarmbands to test their effectiveness. While they increased their natural abilities, they also affected their judgment. They hijacked the Stargate to destroy a new mothership Apophis was building, but the armbands came off halfway through the mission. However, Teal'c was able to rescue them. Later, there were concerns that O'Neill and Carter were compromised as Za'tarcs. It was only after they admitted that they had feelings for each other that they were exonerated. (SG1: "Upgrades", "Divide and Conquer")

"You think the Asgard named a ship after you because they thought it was a cool name? Now's not the time to play dumb, you're a lot smarter than that. They saw our potential in you... because of who you are and what you've done. Humanity's potential."

When researchers in Antarctica found an Ancient frozen in the ice, SG-1 and Dr.Janet Fraiser traveled down to study her. Ayiana, as the Ancient was named, was carrying a plague that infected everyone. While she healed the others, O'Neill was still sick, and Aiyana soon died afterwards. O'Neill was implanted with Kanan, a Tok'rasymbiote only because MajorSamantha Carter practically begged him. After the blending, Kanan had the compulsion to rescue Shallan, the lo'taur of Ba'al, but the mission went wrong and Kanan left O'Neill's body. O'Neill was tortured by Ba'al. While Dr.Daniel Jackson approached him and offered him a chance to ascend, he refused. It was only after Yu attacked the base that O'Neill and Shallan were able to escape, but it left O'Neill with an even deeper distrust of the Tok'ra—and a long lasting animosity toward Ba'al. (SG1: "Frozen", "Abyss")

Shortly afterwards, O'Neill was abducted by the Asgard scientist Loki, who believed that O'Neill was the key to solving their cloning problems. However, O'Neill, while advanced for the Tau'ri evolutionary chain, was not the solution for their cloning problems, and Thor placed a marker in his DNA to prevent manipulation. O'Neill was replaced by a clone who failed to mature at a normal rate, and SG-1 was able to capture Loki. After repairing the clone's DNA, a visibly angry Thor took Loki into his custody. (SG1: "Fragile Balance")

"I wish I could say I didn't owe anything to anyone. But the truth is, I wouldn't be standing here if it wasn't for the courage and support of each and every one of you. I hope I can be as good a leader as we've had in the past and as good as you deserve."

Before he erased the knowledge, Thor interfaced O'Neill's mind with the computer of the Daniel Jackson, and asked him to build a weapon to fight the Replicators. O'Neill constructed a disruptor to sever the Replicator blocks and render them inert, allowing SG-1 to rescue a captured MajorSamantha Carter and save Orilla.

After sending SG-1 to warn Harold Maybourne of the impending approach of Ares, they discovered an Ancient vessel designed to travel in time. As O'Neill was the only one who could fly the vessel, he went off-world to activate it. He was able to destroy Ares' Ha'tak during a battle, killing Ares and saving the people of the planet. The entire experience was rather nostalgic for O'Neill, who commented that he "missed going off world". When the adventure was over, he even said a friendly goodbye to his old rival Maybourne. (SG1: "It's Good to Be King")

General O'Neill asked SG-1 to help Martin Lloyd in making a movie of Wormhole X-Treme!, to insure plausible deniability in case word of the Program was leaked. O'Neill also accompanied SG-1 through Mitchell's 200th trip through the Stargate, having a desire to go through it at least one more time. He convinced Landry to join them. (SG1: "200")

Though O'Neill had ordered Major GeneralHenry Landry to have Daedalus destroy Atlantis should the Ancients lose control of the city, Lt. ColonelJohn Sheppard, Dr.Elizabeth Weir, Dr.Carson Beckett, and McKay hijacked a Puddle Jumper to try to rescue O'Neill, Woolsey, and Atlantis. O'Neill, meanwhile, had to manually drain the water from an underwater Jumper bay to rescue them, resulting in Woolsey and himself being captured. The Replicators mind-probed him, but O'Neill actually managed to resist it due to his previous experiences, surprising the Replicators. When McKay told them his fake plan, O'Neill guessed the truth due to his years with Lt. ColonelSamantha Carter but didn't let on, his ability to resist the mind-probe protecting this from the Replicators as well.

Eventually, Sheppard, Weir, McKay, Beckett, Teyla Emmagan, and Ronon Dex were able to rig the shields to release a Replicator disruptor wave, destroying the Asurans in one fell swoop. When the team contacted the Daedalus and ColonelSteven Caldwell was unsure of them, O'Neill sarcastically addressed him, trying to convince him of the truth. O'Neill's presence and the team's insistence ultimately caused Caldwell to stand down and send a team of Marines to check out their story. On his return to Earth, O'Neill recommended that Weir be placed back in command. (ATL: "The Return, Part 1", "The Return, Part 2")

Jack was called in when Dr.Daniel Jackson was found to be a Prior. After a heated debate, he was convinced to allow SG-1 to capture an Ori warship carrying the incomplete Sangraal, hoping to send it to the Alteran Home Galaxy and neutralize the Ori. After sending Teal'c to interrogate Daniel, O'Neill finally believed his story. Daniel, however, commandeered the Odyssey and used Jack to fly him to the Supergate, as part of his plan. He needed someone to be able to work the Odyssey's controls once he left the ship and someone with authority to give an order so he chose O'Neill, the one person left he trusted. O'Neill was reluctant to trust Daniel due to the kidnapping, but exchanged banter with him.

O'Neill gave the order to the Daedalus in the Pegasus Galaxy to destroy the Stargate linked to the Supergate, allowing Daniel to dial the Supergate and send the weapon through after a moment's hesitation, apparently deciding to trust Daniel. Though they sent the Sangraal, another eight Ori warships entered the Milky Way. Before the ship could travel through the Supergate, O'Neill, at Lt. ColonelSamantha Carter's request, beamed SG-1 back aboard the Odyssey seconds before the Ori ship made the trip. Not knowing it actually worked, he witnessed the Supergate activating, sending an addition six Ori ships to the Milky Way, making the total up to eight.(SG1: "The Shroud")

It was revealed later that O'Neill and SG-1's efforts had succeeded in destroying the Ori. (SG1: "Dominion")

In an alternate reality, Jack O'Neill was a Brigadier General and the commanding officer of the SGA in 1998. As was the case with his counterpart in our reality, he was part of the original team which traveled through the Stargate to Abydos in 1995 with the intention of detonating a nuclear weapon. At the time, he was willing to commit suicide as his son Charlie had recently died after accidentally shooting himself with Jack's personal gun. When the Goa'uld invaded Earth and he discovered the location of Chulak from the Dr.Daniel Jackson of our reality, he sent a nuclear weapon through the Stargate to attempt to halt the attack. He did not believe the Jaffa were worth salvaging a relationship with. This led to his death when he tried to turn Teal'c from his service to Apophis using footage of his counterpart from our reality provided by Daniel. Unfortunately, this resulted in Jack dying with Teal'c personally killing Jack in revenge for Jack sending the bomb to Chulak that had killed Teal'c's wife and young son. The Jack O'Neill from this reality was colder and more militaristic than his counterpart in our reality, purely due to the fact that he had never met Daniel with Daniel helping Jack to mellow and ultimately grow into a better person. He was engaged to Dr.Samantha Carter at the time of his death, indicating that he and Sara O'Neill had divorced by then. It should also be noted that he had a permanent scar on his left eyebrow which was the main timeline O'Neill would receive in the Season 3 episode, "Demons". (SG1: "There But for the Grace of God")

Jack and Sam's wedding in an alternate reality

In another alternate reality, Jack O'Neill had been married to Dr.Samantha Carter for a year by 1999. He died in the Goa'uld invasion of Earth, having been killed by Teal'c, the First Prime of Apophis, in front of his master as well as Sam. His death had a huge emotional impact on his wife. It seems that he was very similar to his counterpart in our reality. It appears that he also frequently used the phrase "for crying out loud" and would not "have a clue" what to tell a woman in the middle of an emotional crisis. The Jack of our reality asked her how she could marry "such a loser." MajorCharles Kawalsky considered Jack his best friend. Consequently, he reacted angrily upon seeing the Teal'c of our reality at Stargate Command, though he eventually came to trust him. (SG1: "Point of View")

In an alternate timeline, Colonel Jack O'Neill was retired and living in his cabin in 2010. He was visited by Samantha Carter when she and DoctorJanet Fraiser discovered the Aschen had sterilized most of the Earth's population. They, DoctorDaniel Jackson and Teal'c developed a plan to prevent this from happening, but they wanted O'Neill's help. At first, he was reluctant, but later joined them. When the plan was put into action, he was the first one to attempt to get a note through the Stargate. He was killed by the automated defense system before reaching it. (SG1: "2010")

In a possible future, the Aschens' attempt to get revenge against the Tau'ri resulted in the creation of a pathogen that destroyed virtually all life on Earth and spread throughout the Stargate network; by 2033, Commander Jack O'Neill was the last living member of Stargate Command, and had a daughter, Jade O'Neill (Her mother was not identified). With the aid of a Reetou ally and Ancient teleportation pods, Commander O'Neill and Jade travelled back in time to prevent the alliance that led to the Aschen Pathogen contaminating Earth, resulting in him meeting his younger self and Daniel. The two O'Neills initially argued about their methods- Commander O'Neill having resorted to the 'whatever-it-takes' approach that Jack had abandoned over his years in the SGC, willing to bomb the treaty negotiations and kill even Tealc and Carter just to save Earth- but reconciled in time to destroy the Aschen ship from the inside before escaping using an Ancient teleportation pod. Commander O'Neill and Jade later returned to the future, accepting whatever would happen to them in favor of a superior future (Stargate SG-1: Relativity)

Jack O'Neill in an alternate timeline

In an alternate timeline accidentally created by SG-1 in which the Stargate Program never existed, Colonel Jack O'Neill was retired and owned his own boat called Homer. In 2005, he went to Chulak with DoctorsDaniel Jackson and Samantha Carter to recruit Teal'c. They traveled back to ancient Egypt to fix the timeline. As he and Sam worked alone in the Puddle Jumper, Jack told her that he found her attractive, but she replied that she liked Daniel, whom Jack thought might have been gay. A short while later, however, when thinking they were going to be killed, she kissed him, saying that she had lied and that she had only wanted to take her time to get to know him better. He then cut off her rambling explanation by kissing her again. When Daniel told them over the radio that it was safe to come outside, Jack replied that they would be a bit longer; they then kissed again, sparks flew from the broken machine in the background, and Sam remarked that she could "fix that.". (SG1: "Moebius, Part 1", "Moebius, Part 2") He and Sam later began a relationship and had a daughter named Eleanor O'Neill, who was named after Sam's late mother Eleanor Carter. (Stargate SG-1: Moebius Squared)

Colonel Jack O'Neill in an alternate timeline

In the alternate timeline created by Ba'al in which the Stargate Program also never existed, Jack O'Neill was a Colonel in the United States Air Force and Charlie O'Neill was still alive in 2008. Prior to history being altered, Jack was killed by the original Ba'al as he attempted to escape back to Earth. Because of SG-1 passing through a wormhole on their way to Earth, they were unaffected by the changes in the timeline were able to restore the correct course of the timeline, leading to Jack not being killed by Ba'al's clone and continuing on with the Extraction Ceremony. (Stargate: Continuum)

Jack has a very dry sense of humor (which he acquired after his mission to Abydos, before that he was a quiet and serious man) and is impatient with complicated explanations. However, in a conversation with Lt. ColonelSamantha Carter, he explains that he is worried about her because she has not tried to baffle him with scientific babble for a few days. When he is asked a scientific question he doesn't know the answer to, he often jokingly answers with "magnets," or some variation of the term. As stated by Cassandra Fraiser, Jack pretends he's not as smart as he really is. (SG1: "Window of Opportunity", "Point of No Return", "Rite of Passage", "Affinity").

At some point, he married Sara O'Neill and together they had a son and named him Charlie O'Neill. Shortly before traveling through the Stargate for the first time, Charlie was killed while playing with his pistol and, in the aftermath of this tragedy, he separated from Sara O'Neill. (Stargate, SG1: "Cold Lazarus")

He had a relationship with CIAAgentKerry Johnson, but she felt there was something deeper between him and Lt. ColonelSamantha Carter. Kerry told him to retire but he didn't take her advice. She goes on saying they were making big error if they let military rules stop their feelings. (SG1: "Threads")

Jack's friendship with Teal'c is one long-standing friendship that has lasted for over ten years and continues to this day.

The two men met each other in grim circumstances during a mission to Chulak where SG-1 later became Apophis's prisoners. After the System Lord had chosen new people to be possessed by the Goa'uld he called his "children", Apophis then ordered for the prisoners to be killed which caused mass panic.

During the rioting, Jack appealed to Teal'c- who had already begun to believe that his God was not a true God- insisting to Teal'c that he could save the people with Teal'c's help. Despite having heard many make that promise before and fail to deliver, Teal'c turned against his fellow Serpent Guards and chose to side with the Tau'ri, noting that Jack was the first person he had met who he believed could fulfil that promise. With Jack's help, he and Teal'c were able to kill most of the guards before Jack launched a staff weapon blast at the wall, creating a hole which gave everyone a chance to escape.

Although Teal'c was reluctant to go, Jack eventually convinced him to join them and thus, their friendship was born. Teal'c also assisted Jack and SG-1 as well as SG-3 long enough for DoctorDaniel Jackson and CaptainSamantha Carter to bring most of the remaining survivors back to Earth. Although Teal'c was later met with some distrust upon arriving at the SGC for the first time, he soon proved his loyalty and was permitted to become part of SG-1 (SG1: "Children of the Gods", "The Enemy Within").

When Teal'c once stated he was a brother to him, Jack reminded him that Teal'c was twice his age; Teal'c conceded that he was a very young brother, then, though it had no importance as he was trying to comfort him while MajorSamantha Carter was missing. (SG1: "Grace")

Over the years, Teal'c and Jack's friendship has continued to grow strong with both men often willing to help the other in times of despair.

Jack and Daniel have a great respect for one another, despite their differences. Ever since the mission to Abydos, they have been on a first name basis, even after Jack was promoted. Although their relationship became more strained after Jack lied to Daniel during his undercover operation and ignored Daniel's advice during a crucial mission, in the aftermath of Daniel's Ascension they became closer once more, Daniel commenting that he was essentially Jack's best friend when talking with an alternate version of Jack (SG1: "Shades of Grey", "The Other Guys", "Meridian", "Fallen", "Continuum").

In the original movie, this character's last name is spelled "O'Neil", with one L. Since O'Neil in the movie and O'Neill in the series are obviously supposed to be the same person, his name is spelled in this article the same way as in SG-1/Atlantis canon, even when speaking about the events of the movie. See Stargate canon for more details about this discrepancy.